In the dynasty league I'm in you keep all 32 players and we have no salary caps or anything. You get em, you keep em. I love this league and wouldn't have it any other way. If you are interested in the rules and such let me know and I'll get you the website. If not, that is fine also.

"Imagine, if you will, that I am an idiot.
Then, imagine that I am also a Congressman.
But, alas, I repeat myself."
Mark Twain

I'm in a 14 player keeper league. We keep 4 players per year. It used to be three, but we upped it.

The big thing in strategy is that you are able to make deals for the future. In the first year I joined (two years ago), I had as my keepers Mussina, Luis Gonzalez, and Chipper Jones. By midseason, it was apparent that I had a middling team, and dealt away Luis and Chipper to get younger players: Helton and Manny Ramirez. I came within one home run of winning it the next year. It keeps more players interested all year long, and more closely models reality.

I'd be intrigued with a full roster keeper league, which would be even closer. But I'd fear that the level of obsession would be too high.

im in a Keeper League.
There are 18 Teams.
30 players per team.
$350 Salary Cap.
15 Keepers Each Season.

so the basic idea is you keep 15 players from one season to another. The other 15 players you didnt keep are sent into the draft. The worst team from the previous year gets the best pick, the best team gets last pick. You draft your 15 players and have a full roster again.
Another important thing is you can only have a player for 3 seasons...it may be different in other leagues but 3 seasons is the max in my league.